[Harper]

To me, it looks a bit like these lines are from the original scan. Something on the back of the music showing through. I wonder if the new PDF renderer is displaying at a higher contrast making these more visible.

Andy, yes, I'd bet it has something to do with the bleed through too but the lines cover more real estate than just the bleed thru area.  The lines are actually displaying as blue on the screen when the bleed thrus are shades of gray.  

Unfortunately I don't always have the ability to re-scan the original since I don't have it.